1. Field of the Invention
The invention relates to a dental milling instrument having a base which supports a working table, the base being connected to a guide column for a vertically adjustable milling arm which receives a milling unit.
2. Description of the Related Art
When milling and drilling operations are performed on dental objects, in particular on dental casts and prosthodontic parts, in addition to the horizontal positioning of the milling or drilling tools, it is necessary also to adapt the vertical position of the tools to the heights of the various workpieces. It is frequently also necessary to reassume a previous, defined vertical position after a height adjustment of the milling arm.
In the dental milling instruments known today, this height adjustment is typically accomplished manually via toothed racks or threaded spindles on a central guide column. In the case of a known dental milling instrument (DE 36 11 518 C2), the guide column is rotatably supported on the base. The milling arm is guided on the guide column so as to be vertically adjustable and capable of being clamped in place by means of a hand lever. The swivelling guide column is capable of being locked on the base by means of a locking screw. With a view to height adjustment, the milling arm has to be displaced manually on the guide column, in which connection a helical screw serves for weight compensation. In order to attain the requisite stiffness and precision for the subsequent milling and drilling operations in a given case, an actuation of the various fixing devices is necessary after the adjustment. The operating elements of these fixing devices are frequently arranged at places on the milling instruments which are spaced far apart from one another, consequently constraining the user to a mode of operation that is not very ergonomic. The clamping forces obtained by such fixing devices depend on the manual force applied by the user, and are consequently undefined, which can lead to inaccuracies in the subsequent working steps.
For various working processes, it may be necessary to regain a defined vertical position after a vertical displacement of the milling arm. To this end, use is generally made of manually adjustable stops on the vertical guide. These mechanical stops must be kept clean at all times, since, for example, metal chips which might fall on them can impair the accuracy of the positioning.
It is therefore an object of the present invention to design a dental milling instrument in such a way that a vertical adjustment can be carried out in a simple manner, without adjusting levers, adjusting handwheels or clamping levers having to be manually actuated for the purpose.
The present invention provides a dental milling instrument with a milling arm which can be displaced along a guideway of the guide column, by means of an electromotive height-adjusting drive.
In the present invention, vertical adjustment is accomplished exclusively by actuation of an electrical switch, for example a pushbutton switch, in an operating panel that can be easily accessed by the user. The vertical fixation can be guaranteed, for example, by the holding moment of the electric adjusting motor.
The height-adjusting drive according to the invention preferably comprises a screw link actuator which can be driven by an electric adjusting motor, and which preferably comprises a threaded spindle which is supported in the base, and which is in engagement with a spindle nut on the milling arm. Hence the electromotive adjusting drive is realised in a device that is simple to construct and saves space.
According to a preferred embodiment, the invention is provided with a screw link actuator of self-locking construction. Hence, a vertical fixation of the milling arm on the guide column is obtained automatically in the currentless state.
With a view to achieving a precise vertical position, it is preferred that the screw link actuator can be set so as to prevent backlash.
The electric adjusting motor according to the invention is preferably a stepping motor. From the number of steps of the motor, the vertical position that is attained in a given case can then be identified and pre-specified by an electric control unit. The control device for the stepping motor supplies a position signal which is derived from the number of steps of the motor, and which corresponds to the vertical position of the milling arm to an indicating and/or positioning device. A set position can be stored in the positioning device of the control device and reassumed at will.
It is possible instead that the milling arm comprises a pickup for a digital, displacement-measuring device which is connected to the guide column, and which supplies a position signal corresponding to the vertical position of the milling arm to the indicating and/or positioning device.
Further preferred configurations of the inventive concept are the subject of the claims of the application. Additional variations and modifications will be apparent to those skilled in the art.